The purpose of this study is to identify, clone and characterize non-HLA genes which are involved in susceptibility to insulin-dependent diabetes mellitus. Previous investigations have demonstrated linkage between IDDM and genetic markers on the long arm of human chromosome 20. Further analysis of this linkage disequilibrium between specific alleles of chromosome 20 polymorphic markers and IDDM susceptibility will require a large number of families with two or more siblings with IDDM.